1. Field of the Invention
The disclosure generally relates oil field equipment. More particularly, the disclosure relates to the blowout preventers.
2. Description of the Related Art
In gas and oil wells, it is sometimes necessary to shear a tubular member disposed therein and seal the wellbore to prevent an explosion or other mishap from subsurface pressures. Typically, the oil field equipment performing such a function is known as a blowout preventer (“BOP”). A BOP has a body that typically is mounted above a well as equipment in a BOP stack.
A typical BOP has a body with a through-bore through which a drill pipe or other tubular member can extend. A pair of rams extend at some non-parallel angle (generally perpendicular) to the through-bore from opposite sides of the bore. The rams are able to move axially within guideways at the non-parallel angle to the bore. A pair of actuators connected to the body at the outer ends of the rams cause the rams to move along the guideway, and close around or shear the drill pipe disposed therebetween. Different types of blades can be coupled with the rams depending on the style of the blowout preventer, and typically include “pipe,” blind, or shear blades. A ram with a blade has one or more sealing surfaces that seal against an object, including an opposing ram. For example, shear blades are typically at slightly different elevations, so that one shear blade passes slightly below the other shear blade to cause the shearing action of a pipe or object disposed between the rams. After the shearing, sealing surfaces on the rams can seal against each other, so that the pressure in the well is contained and prevented from escaping external to the well bore.
In typical BOPs, the shear blades typically are “V-shaped” that contact outer perimeter points of a tubular member disposed in a through-bore opening of the BOP, deforming the tubular member between the opposing V-shaped blades, and shearing the tubular members starting at the lateral outside edges between the V-shaped blades. Typically, the shear blades do not extend to the outer perimeter of the BOP through-bore. The outer perimeter is reserved for sealing members and the structure required to support the sealing members to contain the well bore pressures. Thus, if a tubular member is off-center in the through-bore, the shear blades may bypass the tubular member, and not shear the tubular member. Further, the bypassed member can become lodged between the shear blades and damage or at least block further movement of the shear blades.
A further challenge in typical BOPs is the ability to retrieve the sheared tubular member also termed a “fish.” The fish is created by deforming the tubular member into a substantially flattened shape initially between the shearing blades, and then shearing the tubular member with the BOP. The perimeter of the flattened fish is equal to the perimeter of the prior tubular member. However, the width of the flattened fish across the BOP is wider than the prior diameter of the tubular member, because the flattened fish is smaller in depth compared to the prior diameter. Sometimes, the fish can be difficult to retrieve or can become stuck in the attempt to retrieve.
Therefore, there remains a need for improved blowout preventer to center and shear tubular members disposed therethrough.